How much of this interface needs to actually be
implemented? I've been reading the code, and it seems that at least
some of it could simply remain unsupported while still making ChucK
at least somewhat usable.
Strictly speaking I don't think you need it at all to have a usable ChucK; ChucK was fine before we had it.
Personally I'm a big fan of the Joystick HID interface; I find game controllers to often be very expressive, inspirational in design and relatively affordable compared to commercial controllers meant for music... but you don't actually need any to ChucK.
I wouldn't make porting ChucK dependant on a complete HID game device implementation. The one in ChucK is incomplete as well and doesn't (yet) support HID output (for things like leds, rumble and force-feedback).